FIG. 5 shows a conventional resistance-capacitance oscillation circuit (hereinafter, referred to as an “RC oscillation circuit”) as disclosed in Patent Literature 1 (particularly, FIG. 2(b)). This conventional RC oscillation circuit comprises an amplifier 1 and a phase shifting circuit 2. The phase shifting circuit 2 comprises at least three RC circuit elements 31 each composed of one resistance and one capacitor. In FIG. 5, a first RC circuit element 31a, a second RC circuit element 31b, and a third RC circuit element 31c are provided. As shown in FIG. 5, the first to third RC circuit elements 31a to 31c comprise first to third resistances 41a to 41c and first to third capacitors 51a to 51c. 
The output terminal of the amplifier 1 is connected to one terminal of the first resistance 41a. The other terminal of the first resistance 41a is connected to one terminal of the second resistance 41b through a first node 61a. One terminal of the first capacitor 51a is connected to the first node 61a. The other terminal of the first capacitor 51a is grounded.
The other terminal of the second resistance 41b is connected to one terminal of the third resistance 41c through a second node 61b. One terminal of the second capacitor 51b is connected to the second node 61b. The other terminal of the second capacitor 51b is grounded.
The other terminal of the third resistance 41c is connected to an input terminal of the amplifier 1 through a third node 61c. One terminal of the third capacitor 51c is connected to the third node 61c. The other terminal of the third capacitor 51c is grounded.
When the amplifier 1 is turned on, the RC oscillation circuit oscillates at a predetermined oscillation frequency according to the resistance value of each of resistances 41a to 41c and on the capacitance value of each of capacitors 51a to 51c. 
Patent Literature 2 (which corresponds to US Pre-Grant Patent Application Publication No. 2008/0150646) discloses an RC oscillator using a variable resistance and a variable capacitor.